NSA officials were unable to find any evidence Snowden ever had.
This is essentially the "I do not recall" equivalent of paperwork investigations.
The essential question here is whether the NSA can conclusively deny that Snowden never raised concerns at the agency. Since if he did raise concerns, he probably would have raised them to people personally, a document search is not nessesarily going to uncover whether he did.
What will uncover this conclusively is a simple interview of NSA and affiliate company employees and especially supervisors who worked with Snowden. But since such a set of interviews would either a) reveal that he did raise concerns, b) involve people having to sign their names to untruths, or most unlikely c) reveal he really raised nothing, then I think it's easier for the NSA to just pretend that a half-assed email server word search constitutes an appropriate investigation.